Refined petroleum oils, which are used to manufacture lubricants and other industrial oils, are called lubricant bases or lube oil bases. Lubricants and other industrial oils are produced by mixing lubricant bases with additives, some of which contain metals (Ca, Zn, etc.), which confer them the required qualities for their application (resistance to oxidation, to shearing, and to temperature, emulsifying and anti-foaming qualities, minimum change in viscosity with temperature etc.). The oils, discarded after having been used in engines or other machines, are called used oils. They contain lubricant bases and additives and their breakdown products (lighter petroleum fractions such as naphtha and gas-oil, and heavier ones such as asphalt and coke). They also contain contaminants acquired during their collection from garages and petrol stations, such as water, glycol and solvents.
The separation of asphalts, additives and breakdown products is usually done by vacuum distillation of the oil bases. Said process involves heating the used oil to temperatures above 300° C., which produces cracking reactions that foul the heat exchange and distillation equipment and produce corrosion.
To reduce the equipment fouling, in the separation of asphalts and additives by distillation, several process have been used. Patent WO 9407798 (Viscolube Italiana Spa, 1994), treats the used oil with a strong base before separating the asphalts and additives and carries out this separation by distillation at moderate vacuum (20-30 mbars) and high temperatures (350° C.) under which the additive molecules are broken down. Patent WO 9421761 (Sotulub, Tunez, 1994) carries out a series of treatments with strong bases at 150-250° C. before the separation of asphalt and additives, which in this case is performed in a thin-layer vaporization equipment with very moderate temperatures (310° C.) and a high vacuum (1 mbar) to avoid cracking. Other processes (“The Vaxon Process”, K. Kenton y J. Hedberg, First Intern. Congress on Liquid Waste Refining, May 23 1994, S. Francisco) use a series of flash vaporisations.
All these processes which separate asphalts and additives by distillation, require heating above 300° C. and thus produce lubricant bases with odour, color, acidity, corrosion etc. greater than those of an oil base from first refining, then requiring a final refining step. Traditionally, this final refining step has been done with sulphuric acid and adsorbent clays but this process has been almost abandoned because of its discontinuous character, because it produces sulphonated wastes difficult to manage and because it is costly. For this reason, patents NL 8304023 (KTI, 1985) or EP 574272 (Chem. Eng. Partners, 1993), among others, use catalytic hydrogenation systems, for this final refining step.
Nevertheless, because of the large investment required for catalytic hydrogenation, there is an attempt to search for alternatives. For example, in patent DE 343336 (Buss A. G., 1985), before separation of asphalts and additives by distillation, the oil is treated with alkaline hydroxides at 230-260° C. in a closed reactor, while the patent U.S. Pat. No. 4,834,868 (F. J. Lappin, 1989), carries out the treatment with alkaline hydroxide in the packing of the column used for the separation of asphalts and additives. Caustic treatment at 200-300° C., combined with oxidation, is carried out after the separation of asphalts and additives in patent WO 9826031 (Sotulub, Tunisia, 1994), requiring a final distillation of the lubricant bases after the caustic refining.
As an alternative to the separation of asphalts and additives by vacuum distillation, extraction processes, using liquid solvents, have been developed (solvent de-asphalting). These processes operate at near ambient temperatures, thus avoiding to a large extent the equipment fouling problems and the cracking of asphalts, additives and breakdown products since these are separated before distillation of the lubricant bases. The solvent most commonly used is liquid propane, which is described in several patents, such as BE 873451 (Snam Proggeti Spa, 1979).
In the solvent deasphalting process, propane extracts, by dissolution, preferentially naphtha, gas-oil and lubricant bases and rejects, as raffinate, the asphalts and water, which have low solubility in propane. The raffinate retains most of the additives, breakdown products, asphalts, and all of the water and glycols. After separating the propane by evaporation and recycling it, the extracted lubricant bases are submitted to atmospheric distillation to separate the light products, and to vacuum distillation to separate the gas-oil and lubricant bases. These bases still require a mild refining treatment with clay or hydrogenation to achieve the quality usually reached in the bases of first refining (Patents of Foster Wheeler Corp. U.S. Pat. No. 433,639, application on 16-1-74 and L. E. Cutler and E. T. Cutler, U.S. Pat. No. 3,919,076, Nov. 11, 1975).
Although the previous separation of asphalts and additives, at moderate temperatures by solvent de-asphalting reduce the fouling problems, these still persist because a small proportion of the additives are extracted with propane. For this reason, chemical pretreatments of the used oil, before the solvent deasphalting process, have been introduced. Those pre-treatments use basic compounds and phase transfer catalysts (J. Krzykawski, M. R. Williams, PCT US/99/116600) and increase the efficiency of the separation of additives in the deasphalting process thus reducing the fouling problems, but have not eradicated them altogether.
Current processes of used oil re-refining by extraction with aliphatic solvents (propane etc.) are characterised in that they require the following stages, which are outlined in FIG. 1 (previous technology):
1. De-asphalting with solvents (with or without chemical pretreatment).
2. Separation of the light products by distillation at atmospheric pressure.
3. Separation of the gas-oil and the bases by vacuum distillation.
4. Final refining of the bases (adsorbent clays, hydrogenation etc.).